The present invention relates to lamps and lamp systems for laser pumping. More particularly, the present invention relates to a means for increasing the power supplied to pulsed cesium lamps which are particularly useful for laser pumping.
A pulsed cesium lamp which is particularly efficacious for pumping of neodymium glass or neodymium-YAG lasers is described in U.S. Pat. No. 4,173,728, issued Nov. 6, 1979 to Dr. Harald L. Witting, and assigned to the same assignee as the present invention. The Witting patent discloses a cesium-vapor lamp which can be pulsed at high currents to produce intense light pulses useful for laser pumping and thyristor triggering. For example, typical currents in the cesium lamp have been measured to be as high as 300 amperes for as long as 200 microseconds. Such lamps may be constructed by filling an alumina arc tube with a cesium-mercury amalgam, rather than with the conventional sodium-mercury amalgam employed in high pressure sodium lamps. Furthermore, in the cesium lamp described in the above Witting patent, which is hereby incorporated herein by reference as background material, there is a pool of liquid cesium disposed in the lower portion of the arc tube. In this cesium-vapor lamp, bare tungsten electrodes are employed since cesium adsorbs strongly on bare tungsten metal, thereby producing a strongly-electron-emitting surface. Life tests conducted on the Witting lamp have shown that such lamps can produce in excess of 9 billion pulses with no degradation in light output. In contrast, xenon flash lamps which are widely used for intense light pulses for pumping neodymium lasers yield, at best, about 10 million light pulses. Furthermore, the Witting patent discloses the fact that these cesium-vapor lamps may be employed for pumping both neodymium glass or neodymium-YAG lasers.
Certain lasers, such as neodymium-YAG lasers, are conventionally pumped with light from xenon flash lamps. For example, a slab of laser material is typically enclosed in a highly reflecting dual elliptical cavity which focusses the lamp from two xenon flash lamps onto a slab of lasing material. Many such lasers are employed in high power applications wherein it is necessary to cool the xenon flash lamps, such as by surrounding them with an outer jacket through which cooling water is pumped. Because of such cooling systems, the quartz envelope of xenon flash lamps can be operated at a high average wall loading, such as, for example, 30 watts/cm.sup.2. The peak loading is, of course, much higher than this because the lamp is pulsed on only for several hundred microseconds and then is turned off for a much longer period of time, such as for approximately 0.1 second.
The problems of supplying higher average power to pulsed cesium-vapor lamps is central to the proper understanding of the present invention. In particular, it should be appreciated that for the cesium-vapor lamp, there is a significant problem with increasing the power to the lamp, and thus with increasing the wall loading of the lamp in such a manner so as to remove heat from the alumina arc tube without cooling the arc wall below a temperature of about 500.degree. C. so as to maintain the required cesium vapor pressure in the lamp, which is preferably approximately 70 Torr. Simple water cooling systems such as those employed for conventional xenon flash lamps are inapplicable to the cooling of cesium-vapor lamps. In order to maintain the proper operating temperature of the arc tube walls in previously-developed cesium-vapor lamps, they are provided with outer envelopes. Furthermore, the space between the arc tube and the outer envelope is evacuated and thereby forms a vacuum gap. It is thus seen that water cooling of the outer envelope produces essentially no change in allowable arc tube wall loading, since radiation transfer across the vacuum gap is not at all sensitive to the outer envelope temperature, as long as it is significantly cooler than the arc tube. Direct water cooling of the arc tube itself is not possible because of the necessarily high arc tube wall temperatures which are required. Even though cesium-vapor lamps presently exhibit an average wall loading limit of about 22 watts/cm.sup.2, increases in this limit are highly desirable.
The cesium-vapor lamp described in the Witting patent is similar to high pressure sodium lamps which have been widely employed in the past for outdoor lighting purposes. These two lamp types are linked primarily by the fact that each preferably employs an alumina arc tube. High pressure sodium lamps have employed sintered alumina arc tubes with niobium leads passing through the alumina arc tube end caps. The electrodes of high pressure sodium lamps are made of tungsten which has been coated with a thorium or barium compound to reduce the work function. In high pressure sodium lamps, fill gas comprises metallic sodium, mercury and an inert gas, such as xenon, to aid in starting and which further acts to reduce heat flow from the arc to the arc tube wall. In such lamps the sodium and mercury are placed into the lamp during manufacture in the form of an amalgam pellet. As in cesium-vapor lamps, high pressure sodium lamps also exhibit a maximum wall loading of about 22 watts/cm.sup.2. The actual heat conduction from the arc to the alumina arc tube wall is only approximately 35% of this value, the balance of the energy flow being radiation (visible and infrared) which is radiated through the alumina wall from the arc. Because the efficacy (that is, the lumens/watt output) of a high pressure sodium lamp improves with increasing arc tube wall temperature, the arc tubes are sized so that the alumina wall runs as hot as possible, consistent with long lamp life. This maximum wall temperature is approximately 1,500.degree. K. in commercial lamps. These sodium lamps have evacuated, outer vacuum jackets, the jacket being much larger in diameter than the arc tube. The heat conducted from the arc to the arc tube wall (approximately 8 watts/cm.sup. 2) must thus be radiated from the arc tube wall to the outer jacket and to the external surroundings. Because alumina has a low emissivity in the visible spectrum region, this radiation occurs mainly in the infrared region at wavelengths longer than approximately 3 microns.
In sum, cesium-vapor lamps, which are similar to high pressure sodium lamps in that they employ alumina arc tubes, have been described in the past as being useful for laser pumping, especially laser pumping of neodymium-YAG lasers. However, increasing the average power supplied to the cesium-vapor lamp is difficult because of the present wall loading limit of approximately 22 watts/cm.sup.2. Furthermore, conventional cooling methods and devices for laser lamps are not employable with cesium-vapor lamps because of the necessity of maintaining the arc tube wall temperature at 500.degree. C. or higher.